BOSTON (Reuters) - Republican presidential candidate Mitt Romney said his Mormon religious faith's history of polygamy could trouble American voters but that he too is bothered by it.

The former Massachusetts governor, whose great-grandfather had five wives and whose great-great-grandfather had a dozen, said in an interview to be broadcast on Sunday that the practice banned by the Mormon church in 1890 was "awful."

"That's part of the history of the church's past that I understand is troubling to people," he said, according to comments to be aired on the CBS network's "60 Minutes" television program. Excerpts were released on Thursday.

"I have a great-great grandfather. They were trying to build a generation out there in the desert and so he took additional wives as he was told to do. And I must admit, I can't imagine anything more awful than polygamy," he said.

Romney has raised the most money among Republican candidates but opinion polls show him trailing former New York mayor Rudolph Giuliani and Sen. John McCain, as well as Sen. Fred Thompson, who has not formally entered the race.

The Church of Jesus Christ of Latter-day Saints, as the faith is formally known, distances itself from 30,000 to 60,000 breakaway Mormons in Utah and nearby states who practice polygamy illegally, as well as the many excommunicated Mormons in polygamous marriages who still identify with the faith.

Joseph Smith, founder of the once-isolated sect based in Salt Lake City, Utah, took at least two dozen wives, say historians. His successor, Brigham Young, had about 20.

The practice was officially banned when Washington, angered by its spread, threatened to deny statehood to Utah.

Romney, 60, who has previously called polygamy "bizarre," does not drink, smoke or swear. He is married to his high school sweetheart, which sits better with conservative Christians than Giuliani's two failed marriages and McCain's divorce.

But as he seeks to become the first Mormon U.S. president, he has faced a challenge in courting conservative Christians who often dismiss his religion as a cult but now could decide his political fate.

On Wednesday, he dismissed as "bigoted" comments by civil rights activist Al Sharpton who said on Monday that "those of us who believe in God" will defeat Romney.

Romney is the fifth Mormon to seek the White House. His father, former Michigan governor George Romney, ran in 1968 and the church's founder, Joseph Smith, was shot to death by a mob during his 1844 presidential campaign.